1. Field of the Invention
This invention relates to the multiplex addressing of ferro-electric liquid crystal displays. Such displays may use a chiral smectic, C, I, and F liquid crystal material.
2. Discussion of Prior Art
Liquid crystal display devices commonly comprise a thin layer of a liquid crystal material contained between two glass slides. Electrode structures on the inner faces of these slides enable an electric field to be applied across the liquid crystal layer thereby changing its molecular alignment. Many different types of displays have been made using nematic and cholesteric liquid crystal material. Both these types of material are operated between a field ON state and a field OFF state; i.e. displays are operated by switching a field on and off.
A more recent type of display uses a ferroelectric chiral smectic C, I, and F liquid crystal material in which liquid crystal molecules adopt one of two possible field ON states depending on the polarity of applied field. These displays are thus switched between the two states by pulses of appropriate polarity. In a zero applied field the molecules adopt an intermediate, configuration. Chiral smectic displays offer very fast switching with an amount of bistability. Examples of chiral smectic displays are described in G.B. No. 2,163,273, G.B. No. 2,159,635 (U.S. Pat. No. 4,713,873), G.B. No. 2,166,256 (U.S. Pat. No. 4,722,594), G.B. No. 2,157,451 (U.S. Pat. No. 4,720,173), U.S. Pat. No. 4,536,059, U.S. Pat. No. 4,367,924, G.B. P.A. No. 86 08,114--P.C.T. No. G.B. 87/00,222 (GB 2,209,610 corresponds to U.S. Ser. No. 07/279,553), G.B. P.A. No. 08,115--P.C.T. No. 87/00,221 (GB 2,210,468 corresponds to U.S. Pat. No. 4,969,719), G.B. P.A. No. 08,116--P.C.T. 87/00,220 (GB 2,210,469 corresponds to U.S. Pat. No. 4,997,264).
There are a number of known systems for multiplex addressing chiral smectic displays; see for example article by Harada et al 1985 S.I.D. Paper 8.4 pp 131-134, and Lagerwall et al 1985 I.D.R.C. pp 213-221. In this system a switching pulse is immediately preceeded by an equal and opposite polarity pulse which switches to the opposite state. The purpose of an opposite pulse followed by the wanted switching pulse is to ensure net d.c. at the liquid crystal material. See also GB 2,173,336A (U.S. Pat. No. 4,705,345) and GB 2,173,629 A.
A disadvantage of this system is a reduce switching time. Also the material sometimes fails to switch to the wanted state but stays in an opposite switched state. This gives inverted contrast which under certain conditions could be difficult to control in a complex display.